I want to upgrade my cam to a performer rpm. But that cam needs at least a 9.5:1 comp ratio. I have a 68 coupe with a 289. I know that it is 8.7:1 stock, and its been bored out .030. I need some suggestions for increasing my ratio (the cheapest possibilities please - fairly small budget).
To effectively raise your compression ratio you only have a couple of real options. I am assuming that you don't want to rebuild your engine. If you are going to do that then you can get any compression ratio you want. Assuming that you aren't rebuilding the engine the only practical thing to do would be to reduce the size of the combustion chamber in your cylinder heads to get the compression ratio up. Essentially, you will have to CC the current cylinder heads to see what the volume per chamber is and then figure out what decrease you need in the chambers to achieve the compression ratio that you want. This is done by milling the heads and by where your cylinder heads machinist places the seat of the valve. By placing placing the seat of the valve shallower then the valve is a little deeper into the combustion chamber and thereby effectively reduces the size of the chamber. These two procedures should help you to achieve your desired compression ratio.
What cylinder heads do you currently have? You may want to consider going as close to 10:1 compression ratio as possible for better performance. remember, a cam like the RPM will work better with a performance set of heads, don't put in a hot cam like that and then choke it off with a stock set of heads. You'll just end up tearing the heads off again.
Ill tell you everything ive got. A performer rpm intake, performer rpm heads w/60cc chambers and 1.9 inch intake valves. A set of tri-y headers, and flowmaster 40's dual exhaust. Im currently running the performer plus cam, but it doesnt match that well with the rpm series. So i wanted to get the matching cam for the heads and intake. But i needed at least a 9.5:1 comp ratio. My engine has been bored over .030, and i never took the time to check the comp ratio when it was all together. Now that its apart i cant tell what the ratio is. Im not very knowledgable about ratio's so im not sure if i already had a high enough ratio with these previously mentioned parts and bore. So basically i want to kno whether i can go ahead and put in the cam or i whether i need to increase the ratio in order to properly run the engine, without burning it up.
Usually with these more agressive cams, they feed off the higher compression. If the maufacturer recommends that Compression it would be wise to follow the advice. If the compression isnt enough for the cam then the engine will not perform as well. I think it would be best to have the advised compression when running a cam that requires it.
Since you have every thing torn apart, you can get some flattop pistons installed to bring up your comp ratio. Since you already have performance heads I think the pistons would be the cheaper way of getting higher compression.
I ended up milling .015 off the heads and .015 off the deck height to get my compression corrected. This is with original 60cc chambers and flat tops with 4cc valve reliefs. Static compression is estimated at 9.7:1.
Just buying flat top pistons isn't going to guarantee your ratio will go up. Not all small block flat tops are the same. They've have very different compression heights. A .020 difference in this can make a point diference in the ratio on a 289. Mill your heads and or block, that'll be the easiest way to up the ratio. Do not mill the intake until you have done the final assembly and after you have verified that the intake needs to be milled. Sometimes they do after head/block milling, sometimes they don't.
monkeyman08880, what year is the 289? From the info I have CR ranged from 9.0 to 11.5 on the 289s from 1963-67. Also was it a 2V or 4V motor stock. The original heads most likely had 54.5cc chambers. Below is a good chart for figuring CRs when changing combustion chamber volume.